Various methods now in commercial use for the production of sodium perborate comprise its crystallization from a supersaturated aqueous solution. A high degree of supersaturation can be obtained by reacting a borate such as sodium metaborate, borax or the like with an oxidizing agent such as hydrogen peroxide or sodium peroxide, optionally in the presence of sodium hydroxide and/or a stabilizer such as magnesium silicate.
In practice, if precautions are not taken, the sodium perborate grains, granules or particles obtained will have a wide range in size, often including a large fraction of very fine particles which, when dry, constitute an extremely troublesome source of dust during handling such as in the preparation of powder mixtures. Furthermore, it has been difficult in practice to reproduce a particular range in particle size so that the regulation of powdered compositions prepared therefrom is problematical.
Present applicant has disclosed in French Pat. No. 1,187,352 a continuous process for the production of perborate grains which have a narrow range of size and which are free from dust. This process permits the preparation within a single apparatus and without further screening, of perborate granules falling within a narrow range of size, exemplarily between about 100 and 300 microns or between about 200 and 500 microns, and also having apparent or bulk densities which are relatively high, exemplarily up to about 0.65 Kg./liter. However, to the best knowledge of the applicant no method has been disclosed whereby a corresponding product can be made with substantially higher bulk density, exemplarily in the range of about 0.75 and 1.05 Kg./liter, also being resistant to impact and abrasion.
Subsequent experimental work has been directed to obtaining sodium perborate grains having a very low apparent density approaching that of other pulverulent substances with which the perborate is mixed in preparation of various powder mixtures--the purpose being to avoid a separation of the respective ingredients during different kinds of handling during use. The applicant has disclosed in French Pat. No. 1,436,629 a process for obtaining hollow grains of sodium perborate free from dust and having an apparent density falling in the range from about 0.38 to 0.45 Kg./liter. These grains are also rapidly dissolved in water.
Sodium perborate grains with low bulk density are obtained also according to French Pat. No. 1,229,652 whereby crystallization from highly supersaturated solutions at low temperatures and moderate agitation results in apparent densities between 0.25 and 0.50 Kg./liter. The Certificate of Addition No. 76,697 to this patent discloses the use of a wetting agent during crystallization to improve the speed with which the perborate grains subsequently dissolve in water; however, the apparent density of these grains is low, exemplarily 0.34-0.47 Kg./liter, within the range set forth in the parent patent.
The perborate grains of the above-described prior art, especially those with low apparent densities, are relatively fragile. Having poor resistance to abrasion and impact, they break down during commercial handling with the formation of undesirable dust. This is particularly objectionable when they are transported by such modern methods as by pneumatic conveyors or fluidized bed transportation. At the time of their use in compounding powdered products, in addition to the dust nuisance, their apparent density has been changed in unpredictable and variable manner. In effect, the disadvantages, which the various modifications in crystallization technique have attempted to overcome, have reappeared.